Aims continue to be definition of factors which affect bile flow and bile acid output. Two areas will be considered: Two areas will be considered: biliary permeability and mechanisms of bile acid uptake into the hepatocyte. Abnormalities in both of these areas have been considered contributory to the cholestasis seen with certain forms of medical therapy. Pilot data indicate that biliary permeability is reversibly modulated by a variety of influences such as peptide hormones, feeding/fasting, and 2/3 hepatectomy. Permeability is assessed using a 1 mg. bolus of horseradish peroxidase (HRP) injected into the perfusate of rat livers perfused single-pass. Measurement of biliary HRP activity yields two peaks of activity; an early (3min) peak representing paracellular passage and a late (12 min) peak representing transcellular passage of HRP from plasma to bile. Hormones which elevate (Ca2+) augment paracellular passage, dibutyryl cAMP does not. Histochemical studies indicate access of HRP is across canalicular tight junctions. Agonist transduction pathways will be studied using phorbol esters as well as inhibitors or protein kinase C, calmodulin an calcium-activated neutral proteases. The influence of charge on biliary access will be studied using anionic, cationic and neutral HRP derivatives. Studies of the action of vasopressin on biliary permeability at three levels of bile flow induced by taurodehydrocholate infusion will allow calculation of biliary diffusion constants and reflection coefficients during VP stimulation. Changes in bile flow and anionic (bile acid) and cationic (acetyl procaine amide ethobromide) biliary excretion rate brought about by VP will be sought to assess physiologic relevance of alterations in tight junctional permeability. The effects of fasting/feeding and 2/3 hepatectomy on paracellular permeability will be defined. With reference to bile acid uptake, characteristics of the uptake site will be further defined using synthetic bile acid analogs designed to provide special information about the site. Freshly isolated hepatocytes will be used and deduction made from the kinetics of inhibition of taurocholate uptake by analogs. The nonpolar nature of the sterol site will be further tested by methoxy cholate derivatives. Other compounds will be used to define the importance of bile acid side chain length vs. charge. These studies are a continuation and conclusion of studies under way at the present time.